Learn how to hit a topspin second serve. That's going to be the best solution.

A properly hit topspin serve will require you to take a full swing at the ball and use the racket head speed to put spin to force it down into the court. The topspin will greatly increase your target window above the net without hitting long.

Practice.
Like a flat first serve, a heavy spinning topspin second serve cannot be attained without tons of practice.
Any decent player should be able to swing at least 95% fast and get his second spin serve IN.
That same player might swing the same speed on a flat first attempt, and get maybe 50% of them IN.

Learn how to hit a topspin second serve. That's going to be the best solution.

A properly hit topspin serve will require you to take a full swing at the ball and use the racket head speed to put spin to force it down into the court. The topspin will greatly increase your target window above the net without hitting long.

Funny I was going to say - develop a solid slice second and run with it. My percentages are sooo high with this ace in the hole. Not a topspin...I'd be hitting long all the time

How aggressive you can be with your first serve depends on the reliability and also effectiveness of your second serve. If your second serve is going in 100% of the time and its easily attacked by your opponent, then you can take more chances on your first. However, if your second is faulty (pun) or can be attacked then you need to try and get more first serves in.

I always get a kick out of playing guys who would blast their first serve and never get one in and then power puff the second and you could just lay into it for a return winner almost every time.

With that said, a topspin second serve is a good option also a slice serve can be just as reliable if it has a little more topspin than a typical slice (i believe this is a Kick serve, as in it kicks up and away).

Its a myth you swing as hard as you can at your second serve if you're attempting to hit a topspin serve (or kick, or twist, or topspin-slice, or American twist, or twist kick...whatever lol). You can have an effective and reliable topspin serve without swinging at 100%. What is true is you do swing just as hard, and maybe a little bit harder, at a spin serve as you do at a flat serve, but this doesn't mean 100% of effort.

If you are double faulting a lot, you need to put away any type of flat, low percentage serve and hit two second serves in a row. Even if you don't have a great second serve percentage (let's say you get two-thirds in), you will significant;y cut down the number of double faults. I can't suggest a particular type of second serve since I don't know your game.

Also keep in mind that you can improve your serve percentage with location and not just type of serve. Obviously you can aim for the center of the service box to give yourself lots of margins for error. I like to aim over the low part (center tape) of the net. If you are feeling good one day (i.e., every serve seems to be going in), go for more agressive placement near the lines instead of more pace.

try to get your first serve % up (say 50%+ go in), either by occasionally using a topspin serve as a first serve or by adding some spin to your flat serve to give you margin. the idea of a 1st serve isn't to get an ace or service winner, rather to start the point on a good footing and in control.

Also, develop a solid topspin serve for your 2nd serve so you can rely on it every time on your 2nd serve. It will make double faults a rare occurrence and you will start points at neutral or better even on your 2nd serve.

Obviously, you need to improve your first serve percentage, and learn an effective kick serve to act as a "double insurance". High percentage of first serves and a reliable kick serve will solve the double fault problem.

As seen on TV, every pro tries to hit a hard first serve for a winner or weak return, then hits a heavy topspin of some kind second serve that is much slower but always goes IN.

The kick serve (or topspin serve) is more effective if it has maximum racket head speed just that the bypass of the strings at contact with the ball is from 7 to 1 o'clock positions i.e. you are spinning the ball upward-foward and into the correct service box.

Don't matter what you call it, the rackethead takes a loop similar to a modern forehand, not a straight down and straight up motion.
As long as you hit your serve relatively hard for your level, both first and second, you're doing fine.
But if you have the weakest serves FOR YOUR LEVEL, then it's time for work and improvement.

Pete Sampras hit two first serves, pretty much. I think this is mentally more stable approach and more productive for non-professional player development. Or two second serves, does not matter. Use second serve as another chance to make it right, not to risk doing something completely different.

Pete Sampras hit two first serves, pretty much. I think this is mentally more stable approach and more productive for non-professional player development. Or two second serves, does not matter. Use second serve as another chance to make it right, not to risk doing something completely different.

Actually Sampras didn't. Studies of his serve show he actually changed both his toss and racket path on the second serve to hit more topspin, even though his first serve had plenty of topspin.

Yandell had a good article on this showing that Sampras would toss even more to the left on his second serve and hit a lot more spin. Yandell measured a second serve with more than 5000 rpm.

While I think you need to be cautious trying to toss a lot more left if you are a rh, especially if you're not as young and athletic as Sampras, hitting with a lot more spin is the key to hitting a good second serve.